My understanding is that anyone who says the Shahadah is Muslim. So if they say the Shahadah and don't pray or fast for Ramadan, then I would call them a "non-practicing Muslim". Isn't this fair term? I looked up "kufr" which seems to mean unbeliever. Since we can't read minds, how can we judge this? But we can judge what a person does.
Kufr means disbelief, yes. This term "non-practicing Muslim" is a new term, I guess it would be the nicer way to say Munafiq. Munafiqeen are hypocrites, those who say the Shahadah yet they don't obey Allah in His Commands. We have to still consider them as Muslim since they uttered with their tongue La ilaha illAllah Muhammadur RasoolAllah, and we don't know their heart, only Allah does. But like you said, we judge what a person does. Someone who says "I'm a Muslim, just non-practicing" yet they openly disobey Allah, and they aren't even striving, they just don't care at all, is not a "non-practicing Muslim". Allah knows best.
And this is different than someone who is still striving to pray, to fast, to obey Allah, yet they sin and they make mistakes and they aren't doing everything that Islam prescribes, but they are still striving, they repent, they constantly try. This isn't a "non-practicing Muslim". This is a Muslim, whether they pray five times or not, as long as they are repentful and striving.
Abandoning the prayer is not being a "non-practicing Muslim", abandoning the prayer is kufr. The Messenger of Allah, salaAllahu alayhi wa salam said: "The difference between us and them is Salat (prayer). Whoever neglects it is a kaafir (disbeliever)." It has a Sahih (authentic) isnaad. And the Sahabah believed that a Muslim who has neglected the prayer is a kaafir too.
Actually some major scholars have commented on the issue of neglecting the prayer, out of laziness or anything else without a valid reason.. If you are interested.
So, I still don't understand the term "non-practicing Muslim".
Allah knows best.